🐛 Platform becomes `None` if `--platform` option is given after command

[shnizzedy]

Describe the bug

 Traceback (most recent call last):
  File "/share/█████████/apps/conda/bin/cpac", line 8, in <module>
    sys.exit(run())
  File "/share/█████████/apps/conda/lib/python3.7/site-packages/cpac/__main__.py", line 310, in run
    main(sys.argv)
  File "/share/█████████/apps/conda/lib/python3.7/site-packages/cpac/__main__.py", line 290, in main
    Backends(**arg_vars).run(
  File "/share/█████████/apps/conda/lib/python3.7/site-packages/cpac/backends/__init__.py", line 27, in Backends
    }[platform](**kwargs)
KeyError: None 

To Reproduce Run cpac run with --platform option after run.

Expected behavior

--platform option is respected or a message tells the user where to use the --platform option.

Versions

• C-PAC: 0.3.0
• Container Platform :
  • Docker
  • Singularity

Additional context Related: #2